Studies of paroxysmal nocturnal hemoglobinuria led to the finding that two populations of erythrocytes are present in the patient group. One population is normal with respect to the membrane protein DAF (decay accelerating factor), while another population had no detectable protein. It was this population that lyses in PNH patients. It was shown that precursor cells which give rise to the DAF negative population and which are present in bone marrow in these patients, do not have the PNH phenotype. They do not lyse in acidified serum and have DAF on their surface. Thus, PNH was shown to be a developmental defect rather than a simple clonal abnormality as previously suspected. The nature of the C3 fragment leading to clearance of cells coated with cold agglutinin and incubated in serum were studied. Such cells were cleared by the liver rapidly when injected into the circulation. It was shown that such cells do not have the complement fragment C3b on their surface, but have C3bi on their surface. Thus for the first time in human beings it was shown that C3bi is capable of causing clearance in man.